Field of the Disclosure
The present disclosure relates generally to wireless communication and, more particularly, to directional couplers used in wireless communication.
Description of the Related Art
A directional coupler is a passive device that couples a defined amount of electromagnetic power applied to an input port from a transmission line to an output port in one direction. Directional couplers may be used as power splitters that divide the power received at an input port into portions provided to two or more output ports. They may also be used (in the reverse direction) as power combiners that combine the power received at two or more input ports and provide the combined power to an output port. The most common form of a directional coupler is implemented as a pair of coupled transmission lines that have ports at both ends of a main transmission line and a port at one end of a coupled transmission line. The port at the other end of the coupled transmission line is isolated and receives no power. A transverse electromagnetic (TEM) mode directional coupler can be implemented using two overlying striplines that are positioned proximate to each other. The linear dimension of the coupled portion of the striplines is approximately λ/4, where λ is the wavelength corresponding to the center frequency of the TEM-mode directional coupler. The striplines are positioned within a cavity to form a quasi-coaxial configuration of the inner stripline and the outer cavity.
Large surface current densities on the striplines in TEM-mode directional couplers can generate high temperatures in the striplines, particularly when the TEM-mode directional coupler is used at powers above hundreds of Watts and depending on the cross section of the striplines. The maximum average power rating for the directional coupler may therefore be limited by the ability of the striplines to dissipate heat. For example, the stripline may oxidize when the temperature of the stripline exceeds an oxidation threshold, which may in turn increase the rate of heat dissipation in the stripline and potentially lead to thermal runaway and failure of the directional coupler when operated above a threshold transmission power. Conventional TEM-mode directional couplers dissipate the heat generated by the surface currents via three modes: (1) conduction through the air that separates the inner stripline from the outer cavity and from the coupler to coaxial lines attached to the coupler, (2) radiation from the surfaces of the striplines, and (3) convection in the air surrounding the inner stripline. The three modes of heat dissipation are limited by the structure of the directional coupler, which determines the volume of air available for conduction or convection and the stripline surface area available for radiation. The average power rating of the directional coupler may be increased by increasing the dimensions of the device, but increasing the dimensions degrades the electrical performance of the TEM-mode directional coupler, if a certain cross-sectional size is exceeded.